Solution and solid state nuclear magnetic resonance spectroscopy are being used along with circular dichroism, microelectrophoretic mobility, and fluorescence measurements to study the structure of peptides and small proteins in solution and in the bound state. The peptides that have been investigated include fragments of the integral membrane envelope glycoprotein, gp4l, of human immuno-deficiency virus-1. One highly conserved fragment (828-848), which is in the cytoplasmic tail region of the gp4l, is required for infectivity. We have shown that this fragment binds electrostatically to the surface of model membranes and that it may tether the protein to the cytoplasmic surface of the viral membrane. We have also studied the structure of peptides bound to a monoclonal antibody that represents immunogenic regions of the viral coat protein of Rift Valley Fever Virus.